Method and apparatus for liquefying hydrocarbon streams

ABSTRACT

A method of cooling at least two hydrocarbon streams such as natural gas, the method at least comprising the steps of: (a) providing at least first and second hydrocarbon streams ( 20, 20   a ); (b) passing the first hydrocarbon stream ( 20 ) through one or more first heat exchangers ( 12, 14 ) to provide a first cooled hydrocarbon stream ( 30 ); and (c) passing the second hydrocarbon stream ( 20   a ) through one or more second heat exchangers ( 12   a   , 14   a ) to provide a second cooled hydrocarbon stream ( 30   a ); wherein a refrigerant circuit ( 100 ) provides cooling to the first heat exchanger (s) ( 12, 14 ) and the second heat exchanger (s) ( 12   a   , 14   a ).

The present invention relates to a method and apparatus for liquefying at least two hydrocarbon streams, such as at least two natural gas streams.

Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at a high pressure.

Usually natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stock suitable for liquefaction at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas is then further cooled and expanded through one or more expansion stages to final atmospheric pressure suitable for storage and transportation. The flashed vapour from each expansion stage can be used as a source of plant fuel gas.

The costs in creating and running a liquefying natural gas (LNG) plant or system are naturally high, and a significant part is for the cooling configurations. Any reduction in the energy requirements of the plant or system has significant cost benefit. Reducing any cost of any cooling configuration is particularly advantageous.

U.S. Pat. No. 6,272,882 B1 relates to a process of liquefying a gaseous, methane-enriched feed to obtain a liquefied product. The liquefaction process comprises a number of steps, one of which is to separate the partly-condensed refrigerant for the main heat exchanger into a liquid heavy refrigerant fraction and a gaseous light refrigerant fraction. At least part of the liquid refrigerant fraction is cooled, liquefied and sub-cooled against off-gas removed from a flash vessel used after the main heat exchanger. The process of U.S. Pat. No. 6,272,882 B1 shows a single ‘train’ for liquefaction.

U.S. Pat. No. 6,389,844 B1 relates to a plant for liquefying natural gas. More specifically, a pre-cooled dual heat exchanger, dual refrigerant system. The plant in U.S. Pat. No. 6,389,844 B1 has a liquefaction capacity which is 40 to 60% higher than that of a single liquefaction train, and comprises one pre-cooling heat exchanger, and at least two main heat exchangers. Each main heat exchanger uses a main refrigerant which is separated into a heavy liquid fraction and a light gaseous fraction which are only seen to be cooled in the main heat exchanger, prior to expansion.

It is an object of the present invention to improve the efficiency of a treatment plant or method, in particular of a liquefaction plant or method.

It is a further object of the present invention to reduce the energy requirements of a treatment plant or method, in particular of a liquefaction plant or method.

It is another object of the present invention to provide an alternative method and apparatus for treating a hydrocarbon stream, in particular for liquefying a hydrocarbon stream.

The present invention provides a method of liquefying at least two hydrocarbon streams, such as at least two natural gas streams, the method at least comprising the steps of:

(a) providing at least first and second hydrocarbon streams; (b) passing the first hydrocarbon stream through one or more first heat exchangers to provide a first cooled hydrocarbon stream; (c) passing the second hydrocarbon stream through one or more second heat exchangers to provide a second cooled hydrocarbon stream; and (d) subsequently liquefying said first and second cooled hydrocarbon streams.

A refrigerant circuit provides cooling to the one or more first heat exchangers and the one or more second heat exchangers, by passing separate streams of refrigerant through the one or more first heat exchangers of step (b) and through the one or more second heat exchangers of step (c).

The separate refrigerant streams, after respectively passing through the first heat exchanger(s) the second heat exchanger(s) and/or having exchanged heat with the respective hydrocarbon streams, may be compressed, either commonly or separately.

The compressed refrigerant streams may subsequently be commonly cooled in one or more common coolers. In this way, there can be a reduction in the number of coolers required, reducing capital and running costs.

In a further aspect, the present invention provides an apparatus for liquefying at least two hydrocarbon streams, such as at least two natural gas streams, the apparatus at least comprising:

one or more first heat exchangers to cool a first hydrocarbon stream and to provide a first cooled hydrocarbon stream;

one or more second heat exchangers to cool a second hydrocarbon stream and to provide a second cooled hydrocarbon stream;

at least one liquefaction system arranged to liquefy said first and second cooled hydrocarbon streams; and

a refrigerant circuit comprising at least two separate streams of refrigerant one of which to provide cooling to the one or more first heat exchangers and the other one of which to provide cooling to the one or more second heat exchangers.

The refrigerant circuit may further be provided with at least one compressor for compressing the refrigerant streams, either commonly or separately, after having provided their cooling to the first and second heat exchanger(s).

The refrigerant circuit may further be provided with one or more common coolers for commonly cooling the compressed refrigerant streams.

Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying non-limiting schematic drawings in which:

FIG. 1 is a generalised scheme of part of a liquefaction plant according to one embodiment of the present invention; and

FIG. 2 is a more detailed scheme of the liquefaction plant in FIG. 1.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components, streams or lines.

Described herein are methods and apparatuses wherein two hydrocarbon streams are cooled against a refrigerant in a refrigerant circuit, by passing a stream of the refrigerant through one or more first heat exchangers and passing a separate stream of the refrigerant through one or more second heat exchangers and subsequently compressing the separate streams of the refrigerant using at least one compressor.

By using a, preferably one, refrigerant circuit to provide cooling to the first heat exchanger(s) and the second heat exchanger(s), a reduction in the capital costs and running costs can be provided by commonality of elements and features in a single refrigerant circuit serving the different heat exchangers for the two hydrocarbon streams.

The refrigerant circuit may involve any number of separate lines or streams of refrigerant to cool different hydrocarbon streams, and any number of common elements or features, including compressors, coolers, etc. Some refrigerant streams may be common and some may be separate.

The first and second heat exchangers may be separate, and any alteration in existing first and second heat exchanger arrangements can be avoided to effect the invention.

Embodiments of the invention comprise common cooling of two or more compressed refrigerant streams.

The compressed refrigerant streams may be combined for commonly cooling through the one or more common coolers. Usually, the common or combined cooling will involve the refrigerant of the refrigeration circuit being condensed. The common cooling equates to common heat rejection from the separate refrigerant streams after their compression. One or more other coolers, being separate or integrated, may also be involved or associated with the compressors as is known in the art.

By using some, preferably the majority, of common cooling in the refrigerant circuit, the present invention can reduce the overall energy requirements of a method or plant or apparatus for treating, in particular liquefying, a hydrocarbon stream, and/or make the method, plant or apparatus more efficient and so more economical.

The refrigerant of the refrigerant circuit may be a single component such as propane. Preferably it is a mixed refrigerant based on two or more components, said components preferably selected from the group comprising nitrogen, methane, ethane, ethylene, propane, propylene, butane and pentane.

In a step (b) of the methods described herein, a first hydrocarbon stream is passed through one or more first heat exchangers to provide a first cooled hydrocarbon stream, while in a step (c) a second hydrocarbon stream is passed through one or more second heat exchangers to provide a second cooled hydrocarbon stream.

In embodiments of the present invention, each of the steps (b) and (c) comprises passing the hydrocarbon stream through 2, 3, 4 or 5 first and second heat exchangers, preferably two first heat exchangers and two second heat exchangers.

The first and second cooled hydrocarbon streams could be further treated, for example liquefied.

Preferably, the first and second hydrocarbon streams are feed streams, preferably provided from a single feed stream. Where the first and second hydrocarbon streams are provided in this way, they may be equally or unequally divided; preferably they are the same. The feed stream can be divided by any suitable divider, stream splitter, or similar known in the art.

In general, a feed stream or streams can be liquefied by passing it through at least two cooling stages. Any number of cooling stages can be used, and each cooling stage can involve one or more heat exchangers, as well as optionally one or more steps, levels or sections. Each cooling stage may involve two or more heat exchangers either in series, or in parallel, or a combination of same.

Arrangements of suitable heat exchangers able to cool and liquefy a feed stream such as a hydrocarbon stream such as natural gas are known in the art.

In one arrangement, this involves the two cooling stages comprising a first cooling stage and a second cooling stage, the first stage being preferably a pre-cooling stage to cool below 0° C., and the second stage preferably being a main cryogenic stage to liquefy below −100° C.

In a particular embodiment of the present invention, the method of treating hydrocarbon streams is part of a method of liquefying a hydrocarbon stream such as natural gas from a feed stream, wherein the method of treating comprises a first cooling stage, and there is a subsequent second cooling stage for liquefying the first and second cooled hydrocarbon streams.

The hydrocarbon streams may be any suitable hydrocarbon-containing streams to be liquefied, but they are usually from a natural gas stream obtained from natural gas or petroleum reservoirs. As an alternative the natural gas stream may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.

Usually natural gas is comprised substantially of methane. Preferably the feed stream comprises at least 60 mol % methane, more preferably at least 80 mol % methane.

Depending on the source, the natural gas may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes and pentanes as well as some aromatic hydrocarbons. The natural gas stream may also contain non-hydrocarbons such as H₂O, N₂, CO₂, H₂S and other sulfur compounds, and the like.

If desired, the hydrocarbon streams may be pre-treated before using them in the present invention. This pre-treatment may comprise removal of any undesired components present such as CO₂ and H₂S, or other steps such as pre-cooling, pre-pressurizing or the like. As these steps are well known to the person skilled in the art, they are not further discussed here.

Although the method according to the present invention is applicable to various hydrocarbon feed streams, it is particularly suitable for natural gas streams to be liquefied. As the person skilled readily understands how to liquefy a hydrocarbon stream, this is not further discussed in detail herein.

Further the person skilled in the art will readily understand that after liquefaction, the liquefied natural gas may be further processed, if desired. As an example, the obtained LNG may be depressurized by means of a Joule-Thomson valve or by means of a cryogenic turbo-expander.

The present invention may involve one or more other or further refrigerant circuits, for example in or passing through a first cooling stage. Any other or further refrigerant circuits could optionally be connected with and/or concurrent with the refrigerant circuit for cooling the first and second hydrocarbon streams.

FIG. 1 shows a general arrangement of part of a liquefied natural gas (LNG) plant. It shows an initial feed stream 10 containing natural gas. In addition to methane, natural gas usually includes some heavier hydrocarbons and impurities, e.g. carbon dioxide, nitrogen, helium, water and non-hydrocarbon acid gases. The feed stream 10 has usually been pre-treated to separate out these impurities as far as possible, and to provide a purified feed stock suitable for liquefying at cryogenic temperatures.

The feed stream 10 is divided by a stream splitter 15 to provide first and second hydrocarbon streams 20, 20 a prior to a first cooling stage 2. The feed stream 10 may be divided into any number of hydrocarbon streams, and FIG. 1 shows the division into two hydrocarbon streams by way of preferred example only. The division of the feed stream 10 could be based on any ratio of mass and/or volume and/or flow rate. The ratio may be based on the size or capacity of the subsequent parts of the liquefaction stages or systems or units, or due to other considerations. One example of the ratio is an equal division of the feed stream mass.

In the first cooling stage 2, the first hydrocarbon stream 20 passes through a first set of two first heat exchangers 12, 14 to provide a first cooled hydrocarbon stream 30. The second hydrocarbon stream 20 a passes through a second set of second heat exchangers 12 a, 14 a, which may be identical or different to the first set of first heat exchangers 12, 14, to provide a second cooled hydrocarbon stream 30 a.

The first heat exchangers 12, 14, and second heat exchangers 12 a, 14 a, are provided with cooling by a first refrigerant circuit 100. The first refrigerant circuit 100 has two refrigerant streams 101 and 101 a which separately cool the first heat exchangers 12, 14 and second heat exchangers 12 a, 14 a respectively. After providing their cooling, the refrigerant streams 101, 101 a are passed into one or more separate compressors 32, 32 a, before the compressed refrigerant streams 101 d, 101 e are combined to provide a single stream 101 f for common heat rejection. The single stream 101 f passes through one or more common water and/or air coolers, two of which coolers 34, 34 a are shown in FIG. 1. The (usually) condensed refrigerant stream 101 g is then divided to provide the separate refrigerant streams 101, 101 a for cooling.

The first cooling stage 2 may comprise any number of heat exchangers for each hydrocarbon stream, and the feed stream 10 may be divided into more than two hydrocarbon streams.

The first cooling stage 2 will generally cool the first and second hydrocarbon streams 20,20 a to a temperature below 0° C., and preferably between −20° C. to −60° C.

In FIG. 1, the first and second cooled hydrocarbon streams 30, 30 a pass through a second cooling stage 4, wherein they are liquefied by two separate liquefaction systems, each generally including at least one heat exchanger respectively, to provide separate liquefied streams 40, 40 a respectively. Liquefaction systems and process conditions for liquefaction are well known in the art, and are not described further herein. In FIG. 1, the two liquefaction systems are symbolically represented by liquefaction heat exchangers 16 and 16 a. These are also heat exchangers, but they are referred to as liquefaction heat exchangers merely in order to label them differently (by their function) from the first and second heat exchangers discussed hereinabove.

Each of the liquefaction heat exchangers 16, 16 a in the second cooling stage 4 of the example shown in FIG. 1 uses a refrigerant circuit: the first liquefaction heat exchanger 16 uses a first refrigerant circuit 102, and the second liquefaction heat exchanger 16 a uses a second refrigerant circuit 103. Each of these refrigerant circuits 102, 103 may use the same or different refrigerants. Preferably, each uses the same refrigerant, and more preferably the refrigerant for each of the refrigerant circuits 102, 103 is a mixed refrigerant. The mixed refrigerant may be based on two or more components, preferably selected from the group comprising nitrogen, methane, ethane, ethylene, propane, propylene, butane and pentane.

Generally, the first and second cooled hydrocarbon streams 30, 30 a, are cooled by the second cooling stage 4 to a temperature of at least below −100° C.

The liquefied streams 40 and 40 a are then combined. They may be combined in any known manner, and in any known combination of steps. Such combination of streams may be prior to or after any expansion of any of the liquefied streams 40, 40 a. The combining of the liquefied streams may not require full integration or mixing for their subsequent passage through a gas/liquid separator. Preferably the streams are combined before passing through a gas/liquid separator.

Arrangements for combining streams are known to the person skilled in the art. The example arrangement shown in FIG. 1 is for the combination of the liquefied streams 40, 40 a using a combiner 18 known in the art, to provide a combined liquefied hydrocarbon stream 50. The combiner may be any suitable arrangement, generally involving a union or junction or piping or conduits, optionally involving one or more valves.

The combined liquefied hydrocarbon stream 50 provided by the second cooling stage 4 can pass through a flash valve (not shown) and then on to a gas/liquid separator such as an end flash vessel 22, wherein the liquid stream is generally recovered as a liquefied hydrocarbon product stream 60, and the vapour is provided as a gaseous stream 70. The liquefied hydrocarbon stream 60 is then sent by one or more pumps (not shown) to storage and/or transportation facilities.

FIG. 2 shows a more detailed scheme of the embodiment of the present invention shown in FIG. 1, wherein the feed stream 10 is divided into the first and second hydrocarbon streams 20 a, 20 b, which pass through the two separate but parallel and identical sets of first heat exchangers, 12, 14, and second heat exchangers 12 a, 14 a, as the first cooling stage 2.

Both the sets of first and second heat exchangers 12, 14, 12 a, 14 a are provided with cooling by the one refrigerant circuit 100. The first refrigerant circuit 100 has the two refrigerant lines 101 and 101 a which separately cool the first set of first heat exchangers 12, 14 at two different pressure levels in a manner known in the art, and cool the second set of second heat exchangers 12 a, 14 a at two different pressure levels in a manner known in the art, respectively.

After providing their cooling, the refrigerant streams 101, 101 a are passed into two sets of compressors 36 and 36 a respectively. Each stream of compressed refrigerant is passed through separate water and/or air coolers 38, 38 a respectively, and then combined to form a single refrigerant stream 101 f. The separate coolers 38, 38 a also provide cooling of the compressors 36, 36 a in their recycle operation.

The single refrigerant stream 101 f then passes through a large water and/or air cooler 34, where the majority of the heat in the refrigerant is exchanged by being rejected to ambient, as condensation of the refrigerant takes place. The refrigerant then passes into an accumulator 42 known in the art. From the accumulator 42, a stream of refrigerant passes through a final and usually smaller water and/or air cooler 34 a before being divided into the two refrigerant lines 101 and 101 a.

Preferably, the large cooler 34 provides the same level of cooling as prior art coolers of separate refrigerant circuits used hitherto fro cooling two hydrocarbon streams.

It is noted that not all the cooling of the refrigerant in the first refrigerant circuit 100 is or need be carried out by a common cooling unit or units, such as the large cooler 34. The separate coolers 38 and 38 a will provide some initial cooling, although they are dedicated to their compressors 36 and 36 a to enable their recycling of gas in a manner known in the art. By way of example only, the ratio of cooling power of the large (and common) cooler 34 compared to the cooling power of the compressor coolers 38 and 38 a can be from 5:1 up to 20:1 or more; but preferably approximately a 10:1 ratio. For the present invention, at least the majority of the cooling of the refrigerant in the refrigerant circuit 100 is provided by a common cooler or coolers after recombination of all the separate refrigerant streams (after their provision of cooling to the hydrocarbon streams in the relevant heat exchangers).

The arrangement of the first refrigeration circuit 100 in FIGS. 1 and 2 simplifies the cooling provided to one heat exchanger, or some of the heat exchangers or all of the heat exchangers, of the first cooling stage 2, or any cooling stage, configuration or arrangement. In particular, the arrangements shown in FIGS. 1 and 2 reduce the number of water and/or air units and accumulators required in a first refrigerant circuit 100, which can nevertheless still provide two refrigerant streams for separate sets of heat exchangers. It may be possible to further reduce the number of features regarding the first refrigerant circuit 100 by further combination of coolers, valves and/or compressors, in order to further reduce the capital and running costs of the first refrigerant circuit 100 and/or the first cooling stage 2.

Downstream of the second one 14 of the first heat exchangers 12,14, there is a cooled hydrocarbon stream 30. This stream 30, and the equivalent cooled hydrocarbon stream 30 a from the second set of second heat exchangers 12 a, 14 a of the first cooling stage 2, then pass into two parallel and preferably identical liquefaction heat exchangers 16, 16 a, which form the second cooling stage 4.

The liquefaction heat exchangers 16, 16 a of the second cooling stage 4 are preferably spool-wound or spiral-wound cryogenic heat exchangers, whose operation is known in the art and whose cooling is provided by the second and third refrigerant circuits 102, 103 respectively.

Each of the liquefaction heat exchangers 16, 16 a provides a liquefied hydrocarbon stream 40, 40 a, which streams 40, 40 a are then combined into a combined liquefied hydrocarbon stream 50. After passage through a third heat exchanger 24, the cooled combined liquefied hydrocarbon stream 50 a passes through an expander, and into a gas/liquid separator, being an end flash vessel 22 known in the art. From the end flash vessel 22 there is provided a liquefied hydrocarbon product stream 60, which can then be passed along by a pump 26 to storage and/or transportation, and a gaseous stream 70, which after any heat exchange, may be used as a fuel gas, and/or used in other parts of the LNG plant.

In the example shown in FIG. 2, the first, second and third refrigerant circuits 100, 102, 103 preferably use a mixed refrigerant. The second and third refrigerant circuits 102 and 103 preferably use the same mixed refrigerant.

The mixed refrigerant of each refrigerant circuit may be based on two or more components, more preferably selected from the group comprising nitrogen, methane, ethane, ethylene, propane, propylene, butane and pentane. The average molar weight of the refrigerant in the first refrigerant circuit 100 is preferably higher than the average molar weight of refrigerant in the second and third refrigerant circuits 102 and 103.

For clarity, the second refrigerant circuit 102 will now be described in more detail. From the liquefaction heat exchanger 16, a stream 102 e of vapourised refrigerant is provided, and compressed and cooled by two compressors and two water or air coolers, to provide a cooled refrigerant stream 102 a. This cooled refrigerant stream 102 a then passes through the set of two heat exchangers 12, 14 of one part of the first cooling stage 2, which provides some cooling to the second refrigerant. This further cooled refrigerant stream 102 b is then passed into a gas/liquid separator 46. The separator 46 provides a light refrigerant fraction 102 c, and a heavy refrigerant fraction 102 d, which both pass into the liquefaction heat exchanger 16 to be cooled and expanded to use their cold energy in the liquefaction heat exchanger 16 in a manner known in the art.

Table 1 gives a representative working example of temperatures, pressures and flows of streams at various parts an example process of the present invention referring to FIG. 2.

TABLE 1 Stream Temperature Pressure Mass flow number (° C.) (bar) (kg/s) Phase  10 51.0 92.6 277.7 Vapor  20 51.0 92.6 139.5 Vapor  30 −41.5 89.0 140.0 Vapor  40 −151.4 83.5 140.0 Liquid  50a −156.8 81.0 280.0 Liquid  60 −162.5 1.1 251.6 Liquid  70 −165.1 1.0 28.4 Vapor 102a 46.0 53.3 205.0 Vapor 102b −41.5 49.0 205.0 Mixed 102c −41.6 48.9 36.0 Vapor 102d −41.6 48.9 169.0 Liquid 102e 62.3 19.9 205.0 Vapor 101 41.0 37.8 442.9 Liquid 101f 68.7 38.6 885.9 Vapor

The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims. 

1. A method of liquefying at least two hydrocarbon streams, the method at least comprising the steps of: (a) providing at least first and second hydrocarbon streams; (b) passing the first hydrocarbon stream through one or more first heat exchangers to provide a first cooled hydrocarbon stream; (c) passing the second hydrocarbon stream through one or more second heat exchangers to provide a second cooled hydrocarbon stream; and (d) subsequently liquefying said first and second cooled hydrocarbon streams; wherein a refrigerant circuit provides cooling to the one or more first heat exchangers and the one or more second heat exchangers, by passing separate streams of refrigerant through the one or more first heat exchangers of step (b) and through the one or more second heat exchangers of step (c), and subsequently compressing the refrigerant streams and commonly cooling the compressed refrigerant streams in one or more common coolers.
 2. A method as claimed in claim 1, wherein step (b) comprises passing the first hydrocarbon stream through 2, 3, 4 or 5 first heat exchangers; and wherein step (c) comprises passing the second hydrocarbon stream through 2, 3, 4 or 5 second heat exchangers.
 3. A method as claimed in claim 1, wherein said subsequently compressing the refrigerant streams comprises separately compressing each separate stream of refrigerant.
 4. A method as claimed in claim 3, further comprising combining the separately compressed streams of refrigerant prior to said commonly cooling in said one or more common coolers.
 5. A method as claimed in claim 1, wherein the passing of steps (b) and (c) forms part of a first cooling stage, and wherein the subsequently liquefying of step (d) comprises further cooling said first and second cooled hydrocarbon streams in a second cooling stage.
 6. A method as claimed in claim 1, wherein the first and second cooled hydrocarbon streams in step (d) are liquefied as separate streams.
 7. A method as claimed in claim 1, wherein the first and second hydrocarbon streams are feed streams.
 8. Apparatus for liquefying at least two hydrocarbon streams, the apparatus at least comprising: one or more first heat exchangers to cool a first hydrocarbon stream and to provide a first cooled hydrocarbon stream; one or more second heat exchangers to cool a second hydrocarbon stream and to provide a second cooled hydrocarbon stream; at least one liquefaction system arranged to liquefy said first and second cooled hydrocarbon streams; and a refrigerant circuit comprising at least two separate streams of refrigerant one of which to provide cooling to the one or more first heat exchangers and the other one of which to provide cooling to the one or more second heat exchangers; at least one compressor for compressing the refrigerant streams; and one or more common coolers for commonly cooling the compressed refrigerant streams.
 9. Apparatus as claimed in claim 8, further comprising a stream splitter to divide a feed stream into at least the first and second hydrocarbon streams.
 10. Apparatus as claimed in claim 8, further comprising at least one separate compressor in each of the separate streams of refrigerant.
 11. Apparatus as claimed in claim 10, further comprising a combiner arranged downstream of the separate compressors and upstream of the one or more common coolers for combining the separate compressed refrigerant streams.
 12. A method of liquefying at least two hydrocarbon streams, the method at least comprising the steps of: (a) providing at least first and second hydrocarbon streams; (b) passing the first hydrocarbon stream through one or more first heat exchangers to provide a first cooled hydrocarbon stream; (c) passing the second hydrocarbon stream through one or more second heat exchangers to provide a second cooled hydrocarbon stream; and (d) subsequently liquefying said first and second cooled hydrocarbon streams; wherein a refrigerant circuit provides cooling to the one or more first heat exchangers and the one or more second heat exchangers, by passing separate streams of refrigerant through the one or more first heat exchangers of step (b) and through the one or more second heat exchangers of step (c), separately compressing each separate stream of refrigerant after passing through the at least one or more first heat exchangers and respectively the at least one or more second heat exchangers, and combining the compressed separate streams of refrigerant.
 13. Apparatus for liquefying at least two hydrocarbon streams, the apparatus at least comprising: one or more first heat exchangers to cool a first hydrocarbon stream and to provide a first cooled hydrocarbon stream; one or more second heat exchangers to cool a second hydrocarbon stream and to provide a second cooled hydrocarbon stream; at least one liquefaction system arranged to liquefy said first and second cooled hydrocarbon streams; and a refrigerant circuit comprising at least two separate streams of refrigerant one of which to provide cooling to the one or more first heat exchangers and the other one of which to provide cooling to the one or more second heat exchangers and at least one separate compressor in each of the separate streams of refrigerant and a combiner arranged downstream of the separate compressors for combining the compressed separate streams of refrigerant.
 14. A method as claimed in claim 2, wherein step (b) comprises passing the first hydrocarbon stream through two first heat exchangers; and wherein step (c) comprises passing the second hydrocarbon stream through two second heat exchangers.
 15. A method as claimed in claim 2, wherein said subsequently compressing the refrigerant streams comprises separately compressing each separate stream of refrigerant.
 16. A method as claimed in claim 14, wherein said subsequently compressing the refrigerant streams comprises separately compressing each separate stream of refrigerant.
 17. A method as claimed in claim 15, further comprising combining the separately compressed streams of refrigerant prior to said commonly cooling in said one or more common coolers.
 18. A method as claimed in claim 16, further comprising combining the separately compressed streams of refrigerant prior to said commonly cooling in said one or more common coolers.
 19. A method as claimed in claim 2, wherein the passing of steps (b) and (c) forms part of a first cooling stage, and wherein the subsequently liquefying of step (d) comprises further cooling said first and second cooled hydrocarbon streams in a second cooling stage.
 20. A method as claimed in claim 3, wherein the passing of steps (b) and (c) forms part of a first cooling stage, and wherein the subsequently liquefying of step (d) comprises further cooling said first and second cooled hydrocarbon streams in a second cooling stage. 